1. Field of the Invention
The present invention relates to a photosensitive device, as well as a photosensitive device having internal circuitry, for use in an optical pickup or the like which supports write operations.
2. Description of the Related Art
Optical pickups are employed for optical disk apparatuses, e.g., CD-ROM or DVD (digital video disk) apparatuses. In recent years, optical disk apparatuses have been increasing in operation speed, and there has been a trend for processing a large amount of data, e.g., moving image data, at higher speeds. Against such a background, there has been an intense demand for increasing the operation speed of optical pickups.
Over the past years, optical disk apparatuses which are capable of writing data on optical disks, e.g., CD-R/RW and DVD-R/RAM, have been developed. Such an optical disk apparatus which is capable of write operations writes information on an optical disk by causing a phase change in a dye which is provided on the disk based on laser-induced heat. High power laser light is radiated onto an optical disk, and light reflected therefrom is incident on a photodiode. Therefore, a much greater amount of laser light is radiated onto the photodiode at the time of writing than at the time of reading. Such writable optical disk media have also been subjected to intense demands for faster operation.
FIGS. 1A and 1B illustrate the structure of a conventional photodiode 1000 which is disclosed in Japanese Laid-Open Publication No 9-153605. As shown in FIG. 1A, this photodiode 1000 includes an epitaxial layer 85 of a second conductivity type provided on a semiconductor substrate 84 of a first conductivity type. The epitaxial layer 85 of the second conductivity type is subdivided into a plurality of regions by diffusion layers 87 and 88 of the first conductivity type. A junction between each subdivided region and the underlying portion of the semiconductor substrate 84 of the first conductivity type provides the photodiode 1000.
The response speed of the conventional photodiode 1000 of the aforementioned structure is a function of a CR time constant, which in turn is a function of the capacitance (C) and the resistance (R) of the photodiode, and the migration distance of carriers, which are generated on the side of a depletion layer 86 closer to the substrate 84, migrating via diffusion.
Therefore, according to this conventional technique, the impurity concentration within the semiconductor substrate 84 of the first conductivity type is prescribed at a low level as shown in FIG. 1B, which illustrates the impurity concentration profile of a cross-section along line I-Ixe2x80x2 in FIG. 1A, so as to obtain a large expanse of the depletion layer 86 within the semiconductor substrate 84 of the first conductivity type. As a result, the junction capacitance of the photodiode 1000 is reduced, thereby decreasing the CR time constant and hence increasing the response speed of the photodiode 1000. Furthermore, since the depletion layer 86 extends deeply into the substrate 84, carriers which are generated at relatively deep portions within the substrate 84 will not have to travel over a large distance via diffusion, thereby also increasing the response speed of the photodiode 1000.
The C component in the CR time constant, which determines the response speed of the photodiode, can also be reduced by increasing the resistivity of the substrate 84 up to a certain value. Thus, as shown in FIG. 2, the response speed (i.e., cut off frequency) of the photodiode can be improved until the resistivity of the substrate 84 reaches that value. However, increasing the resistivity of the substrate 84 further above that value will result in an increase in the serial resistance of the anode side (which contributes to an increase in the R component), so that the response speed of the photodiode, as a function of the CR time constant, is decreased rather than increased, as shown in FIG. 2.
Accordingly, in order to further enhance the response speed of a photodiode, Japanese Laid-Open Publication No. 61-154063, for example, proposes a photosensitive device 2000 having a structure as shown in FIG. 3, where a photodiode is constructed on a laminate substrate obtained by forming a P-type high resistance crystal growth layer 142 on a P-type low resistance substrate 141.
The photosensitive device 2000 shown in FIG. 3 includes an N-type epitaxial layer 143, a P-type separation diffusion layer 144, an N-type contact region 145, an N-type embedded region 146, a P-type base region 147, an N-type emitter region 148, a silicon oxide film 149, electrode wiring layers 150a, 150b, and 150c, a photodiode structural portion 180 for detecting signal light, and a circuit structural portion 190 for processing a detected signal.
A high resistance crystal growth layer 142 includes an autodope layer 142a, which has a gradually decreasing impurity concentration beginning from the low resistance substrate 141, and a layer 142b, which has a constant impurity concentration. According to this conventional technique, the high resistance crystal growth layer 142 makes it easy for the depletion layer 160 to expand into the substrate 141, thereby reducing the junction capacitance. Furthermore, the serial resistance on the anode side is reduced by the P-type low resistance substrate 141, which lies much below the expanse of the depletion layer 160. As a result, both the C component and the R component (which determine the response speed) of the photodiode are reduced, thereby enhancing the response speed of the photosensitive device 2000.
In order to improve the response speed of a photodiode by employing the aforementioned laminate substrate, it is necessary to reduce the junction capacitance by allowing the depletion layer 160 to adequately expand into the high resistance crystal growth layer 142. Therefore, it is desirable to increase the resistivity of the high resistance crystal growth layer 142 up to 1000 xcexa9cm, which corresponds to the maximum controllable resistivity under epitaxial growth, and to prescribe the thickness of the high resistance crystal growth layer 142 at about 20 xcexcm (where the constant impurity concentration portion 142b of the high resistance layer would be about 13 xcexcm thick) so that the depletion layer 160 fully expands in the constant impurity concentration portion 142b of the high resistance layer. Any increase in the region into which the high resistance crystal growth layer 142 does not expand would cause an increase in the serial resistance on the anode side, which in turn prevents the improvement of response speed.
In the case of an optical pickup which supports write operations, the amount of light which is radiated by a laser onto an optical disk increases in proportion with the writing speed, whereby the amount of laser light which is reflected from the optical disk and enters the photodiode also increases. If the amount of light entering the photodiode exceeds a certain level, the response speed of the photodiode may deteriorate.
FIG. 4 shows the dependency of the response speed (i.e., cut off frequency) of the photodiode having the structure shown in FIG. 1 on the incident light amount. As seen from FIG. 4, the response speed (i.e., cut off frequency) of the photodiode decreases as the amount of light entering the photodiode exceeds a certain level. It can also be seen that such a decrease in the response speed becomes more prominent as the resistivity of the substrate increases.
According to the present invention, there is provided a photosensitive device including: a semiconductor substrate of a first conductivity type; a semiconductor layer of the first conductivity type formed on the semiconductor substrate and having a lower impurity concentration than that of the semiconductor substrate; a semiconductor layer of a second conductivity type formed on the semiconductor layer of the first conductivity type; and at least one diffusion layer of the first conductivity type formed from the surface of the semiconductor layer of the second conductivity type so as to reach the surface of the semiconductor layer of the first conductivity type, the at least one diffusion layer subdividing the semiconductor layer of the second conductivity type into a plurality of semiconductor regions of the second conductivity type, wherein at least one photodiode portion for converting signal light into an electrical signal is formed at a junction between at least one of the plurality of semiconductor regions of the second conductivity type and the semiconductor layer of the first conductivity type, and wherein a depletion layer which is formed in the semiconductor layer of the first conductivity type when a reverse bias voltage is applied to the at least one photodiode portion has a field intensity of about 0.3 V/xcexcm or more.
In one embodiment of the invention, the depletion layer formed in the semiconductor layer of the first conductivity type has a thickness of about 5 xcexcm or more.
In another embodiment of the invention, the semiconductor layer of the first conductivity type has a thickness of about 13 xcexcm to about 17 xcexcm and a resistivity of about 100 xcexa9cm to about 1500 xcexa9cm.
In still another embodiment of the invention, the semiconductor substrate has a resistivity of about 1 xcexa9cm to about 20 xcexa9cm.
In still another embodiment of the invention, the photosensitive device further includes: a first electrode provided on a back face of the semiconductor substrate: a second electrode provided on the surface of the semiconductor layer of the second conductivity type, wherein the first and second electrodes are electrically coupled to each other.
In still another embodiment of the invention, the plurality of semiconductor regions of the second conductivity type include at least one first region defining the at least one photodiode portion and at least one second region discrete from the at least one first region, and a signal processing circuitry portion for processing the electrical signal is provided in the at least one second region.
In still another embodiment of the invention, the photosensitive device further includes a high concentration diffusion layer of the first conductivity type formed at an interface between the at least one second region and the first semiconductor layer of the first conductivity type.
Alternatively, there is provided a photosensitive device including: a semiconductor substrate of a first conductivity type; a first semiconductor layer of the first conductivity type formed on the semiconductor substrate and having a higher impurity concentration than that of the semiconductor substrate; a second semiconductor layer of the first conductivity type formed on the first semiconductor layer of the first conductivity type and having a lower impurity concentration than that of the semiconductor substrate; a semiconductor layer of a second conductivity type formed on the second semiconductor layer of the first conductivity type; and at least one diffusion layer of the first conductivity type formed from the surface of the semiconductor layer of the second conductivity type so as to reach the surface of the second semiconductor layer of the first conductivity type, the at least one diffusion layer subdividing the semiconductor layer of the second conductivity type into a plurality of semiconductor regions of the second conductivity type, wherein at least one photodiode portion for converting signal light into an electrical signal is formed at a junction between at least one of the plurality of semiconductor regions of the second conductivity type and the second semiconductor layer of the first conductivity type.
In one embodiment of the invention, a depletion layer which is formed in the second semiconductor layer of the first conductivity type when a reverse bias voltage is applied to the at least one photodiode portion has a field intensity of about 0.3 V/xcexccm or more.
In another embodiment of the invention, the second semiconductor layer of the first conductivity type has a thickness of about 9 xcexcm to about 17 xcexcm and a resistivity of about 100 xcexa9cm to about 1500 xcexa9cm.
In still another embodiment of the invention, the semiconductor substrate has an impurity concentration equal to or smaller than about 1/100 of a peak impurity concentration in the first semiconductor layer of the first conductivity type.
In still another embodiment of the invention, the semiconductor substrate is produced by a CZ method and has a resistivity of about 20 xcexa9cm to about 50 xcexa9cm.
In still another embodiment of the invention, the first semiconductor layer of the first conductivity type has a peak impurity concentration of about 1xc3x971017 cmxe2x88x923 or more.
In still another embodiment of the invention, the first semiconductor layer of the first conductivity type is formed by being applied and diffused.
In still another embodiment of the invention, the first semiconductor layer of the first conductivity type has a region of an increasing impurity concentration from the semiconductor substrate toward the surface of the semiconductor layer of the second conductivity type, and a portion having about 1/100 of the highest impurity concentration throughout the first semiconductor layer of the first conductivity type exists at a depth of about 38 xcexcm or less from the surface of the semiconductor layer of the second conductivity type.
In still another embodiment of the invention, the plurality of semiconductor regions of the second conductivity type include at least one first region defining the at least one photodiode portion and at least one second region discrete from the at least one first region, and a signal processing circuitry portion for processing the electrical signal is provided in the at least one second region.
In still another embodiment of the invention, the photosensitive device further includes a high concentration diffusion layer of the first conductivity type formed at an interface between the at least one second region and the second semiconductor layer of the first conductivity type.
Thus, the invention described herein makes possible the advantages of (1) providing a photosensitive device for use in an optical pickup which supports write operations such that the photosensitive device provides an improved response speed both when receiving a small amount of light during a read operation and when receiving a large amount of light during a write operation; and (2) providing such a photosensitive device having internal circuitry.
Hereinafter, the effects of the present invention will be described.
A photosensitive device according to the present invention incorporates a laminate substrate which includes a semiconductor substrate of a first conductivity type and a semiconductor layer of the first conductivity type formed thereon and having a lower impurity concentration than that of the semiconductor substrate. As a result, the capacitance and the anode resistance of the photodiode are reduced, thereby improving the response speed at the time of receiving a small amount of light during a read operation. In order to prevent a decrease in the response speed due to a flattened potential distribution which would conventionally occur when receiving a large amount of light during a write operation, the depletion layer thickness is controlled by reducing the thickness of the semiconductor layer of the first conductivity type, thereby enhancing the field intensity within the depletion layer. The field intensity within the depletion layer is prescribed at about 0.3 V/xcexcm or more in view of the response speed requirement at the time of receiving a large amount of light (e.g., during a 6xc3x97 speed write operation), which must be satisfied by a photodiode that supports write operations. Furthermore, the depletion layer thickness is prescribed at about 5 xcexcm or more in view of the response speed requirement at the time of receiving a small amount of light (e.g., during a 32xc3x97 speed read operation), which must be satisfied by a photodiode that supports write operations.
In order to satisfy such requirements, it is preferable that the semiconductor layer of the first conductivity type has a thickness (including that of an autodope layer) of about 13 xcexcm to about 17 xcexcm and a resistivity of about 100 xcexa9cm to about 1500 xcexa9cm.
In order to improve the response speed of the photodiode by reducing the anode resistance, it is preferable to minimize the resistivity of the substrate. However, an excessively small resistivity of the substrate would result in autodoping of impurities from the substrate into the semiconductor layer of the first conductivity type during a crystal growth process of the semiconductor layer of the second conductivity type, thereby decreasing the response speed. Such influence of an autodope layer can be minimized to a negligible level by ensuring that the semiconductor substrate has a resistivity of about 1 xcexa9cm to about 20 xcexa9cm.
By providing an anode electrode on the back face of the semiconductor substrate, the anode electrode being electrically coupled to an anode electrode provided on a separation diffusion region located near the surface, the anode resistance can be further reduced as compared to the case where an anode electrode is provided only at the surface of the photosensitive device.
According to another aspect of the invention, a laminate substrate is employed which includes a semiconductor substrate of a first conductivity type and a first semiconductor layer of the first conductivity type formed thereon and having a higher impurity concentration than that of the semiconductor substrate, and which additionally includes a second semiconductor layer of the first conductivity type formed on the first semiconductor layer of the first conductivity type and having a lower impurity concentration than that of the semiconductor substrate. Since the first semiconductor layer of the first conductivity type can serve as a potential barrier as viewed from the substrate, carriers generated on the substrate side of the first semiconductor layer of the first conductivity type cannot override the first semiconductor layer of the first conductivity type to reach the PN junction, and will disappear through recombination within the substrate. As a result, a slow current component associated with the carriers generated within the substrate can be eliminated, thereby increasing the response speed. Since the concentration difference between the first semiconductor layer of the first conductivity type and the second semiconductor layer of the first conductivity type is increased, the field intensity is enhanced, whereby the response speed is improved.
The field intensity within the depletion layer is preferably prescribed at about 0.3 V/xcexcm or more in view of the response speed requirement at the time of receiving a large amount of light (e.g., during a 12xc3x97 speed write operation), which is required of a photodiode that supports write operations.
Furthermore, the thickness of the second semiconductor layer of the first conductivity type is preferably prescribed at a value between about 9 xcexcm and about 17 xcexcm, and the resistivity of the second semiconductor layer of the first conductivity type is preferably prescribed at a value between about 100 xcexa9cm and about 1500 xcexa9cm, in view of the response speed requirement at the time of receiving a large amount of light (e.g., during a 12xc3x97 speed write operation), which is required of a photodiode that supports write operations. The depletion layer thickness is preferably about 3 xcexcm or more, in view of the response speed requirement at the time of receiving a small amount of light.
By prescribing the impurity concentration in the semiconductor substrate of the first conductivity type at about 1/100 or less of the peak impurity concentration within the first semiconductor layer of the first conductivity type, it becomes possible to sufficiently reduce the number of carriers generated on the substrate side of the first semiconductor layer of the first conductivity type at the time of receiving a large amount of light which override the first semiconductor layer of the first conductivity type to reach the PN junction.
It is preferable to produce the semiconductor substrate of the first conductivity type by using a CZ method, which is unlikely to produce defective products. By prescribing the resistivity of the substrate at the highest possible resistivity that can be obtained by a CZ method, e.g., about 20 xcexa9cm to about 50 xcexa9cm, it is possible to increase the level of the potential barrier on the substrate side in the first semiconductor layer of the first conductivity type. As a result, it becomes possible to sufficiently reduce the number of carriers generated within the substrate which override the first semiconductor layer of the first conductivity type, thereby increasing the response speed.
It is preferable that the first semiconductor layer of the first conductivity type has a peak impurity concentration of about 1xc3x971017 cmxe2x88x923 or more, so as to retain a sufficiently high impurity concentration (100 times or greater) relative to the substrate.
It is preferable that the first semiconductor layer of the first conductivity type is produced by being applied and then diffused, which technique is unlikely to produce defective products.
In a region within the first semiconductor layer of the first conductivity type where the increasing impurity concentration increases from the semiconductor substrate toward the surface, it is effective, for the sake of improving the response speed, to ensure that about 1/100 of the highest impurity concentration throughout the first semiconductor layer of the first conductivity type exists at a depth of about 38 xcexcm or less from the surface.
A photosensitive device having internal circuitry according to the present invention includes a signal processing circuitry portion for processing a detected signal that is provided in a region of the semiconductor layer of the second conductivity type separated from the photodiode portions by diffusion layers of the first conductivity type. As a result, the overall pickup system can be downsized.
By providing a high concentration diffusion layer of the first conductivity type below the signal processing circuitry portion and the P-type separation diffusion layer coupled to an anode electrode in the vicinity of the photodiode portion, beginning from the surface of the first or second semiconductor layer of the first conductivity type, it is possible to prescribe a low anode resistance which is required for achieving the response speed at the time of receiving a large amount of light, as a requirement for a photodiode that supports write operations. It also becomes possible to prevent a latch-up phenomenon of the circuitry.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.